1. Field of the Invention
The present invention relates to a waveguide-type optical device using an electro-optic effect, and in particular, relates to a waveguide-type optical device in which an electric field acts on a pair of optical waveguides formed near a surface of a dielectric substrate, in a direction substantially perpendicularly to the substrate.
2. Description of the Related Art
Crystals having the electro-optic effect, particularly ferroelectric crystals such as lithium niobate (LiNbO3:LN), have been applied to many optical devices such as optical modulators, optical switches, and optical attenuators, due to their high electro-optic constant. For example, an LN modulator used in large-capacity, high-speed communication is advantageous from the standpoint of small wavelength chirp, as compared with a modulator having a configuration in which a light source is directly modulated, or an electroabsorption modulator integrated laser diode (EML).
Specifically, as a conventional LN modulator, for example as shown in FIG. 16 to FIG. 18, a variable chirp configuration in which a Z-cut LN substrate is used to form a dual-drive type electrode (FIG. 16), a fixed chirp configuration in which a Z-cut LN substrate is used to form a single-drive type electrode (FIG. 17), and a zero chirp configuration in which an X-cut LN substrate is used to form a single-drive type electrode (FIG. 18) are known (see for example, Japanese Unexamined Patent Publication No. 2004-219521, paragraph numbers 0002 to 0003). Reference numeral 101 in each figure denotes an LN substrate, 110 denotes a Mach-Zehnder optical waveguide, 121 denotes a signal electrode, 122 denotes a ground electrode, and 130 denotes a buffer layer.
It has been reported that, among the above described various configurations, the single-drive type LN modulator is user-friendly, and particularly, the chirp type LN modulator (FIG. 17) with chirps of a required amount, is advantageous in long-distance optical transmission, and this is widely used in long-distance, large-capacity optical transmission systems.
However, the above LN modulators have a larger voltage at the time of operation than that of the aforementioned direct modulation method and the EML, and hence, it is a problem to achieve low voltage (improvement of modulation efficiency) from the standpoint of power consumption and drive amplitude of a driver circuit. This problem is not only in the LN modulator, but is common to various types of waveguide-type optical devices using a substrate having the electro-optic effect.
The waveguide-type optical device having the single-drive type electrode structure using the Z-cut substrate as shown in FIG. 17 is specifically explained. As shown in the X-X′ cross-section in FIG. 19, relative to two optical waveguides formed approximately parallel with each other with a necessary distance therebetween near the surface of the Z-cut substrate, a signal electrode is arranged above one of the optical waveguides and a ground electrode is arranged above the other optical waveguide. In such an electrode structure, an electric field acting on the respective optical waveguides becomes substantially perpendicular to the substrate surface. Curved arrows in FIG. 19 indicate lines of electric force between the signal electrode and the ground electrode. As seen from the state of these lines of electric force, in the single-drive type electrode structure using the Z-cut substrate, the lines of electric force are likely to be dispersed, and in order to obtain a desired electro-optic effect, a relatively high drive voltage needs to be applied to the signal electrode.